A day after Diwali, Gurugram was the least polluted among the NCR cities. However, the deterioration in air quality remained conspicuous. At 2am on Thursday, Gurugram recorded its highest measure of particulate matter 2.5 (PM2.5) this season at 622μg/m3.

Bursting of firecrackers on Diwali sent air quality in the city plummeting, with Gurugram recording 389 (‘very poor’) on the Air Quality Index on November 8, worse from 212 (‘poor’) recorded the previous day.

Similar spikes were seen across the National Capital Region, with Delhi’s AQI worsening from 281 to 390, Faridabad from 262 to 455, Noida from 288 to 432, and Ghaziabad from 295 to 422. A day after Diwali, Gurugram was the least polluted among these cities. However, the deterioration in air quality remained conspicuous. At 2am on Thursday, Gurugram recorded its highest measure of particulate matter 2.5 (PM2.5) this season at 622μg/m3. Just a few hours earlier, at 6pm on November 7, PM2.5 was recorded at 59μg/m3, indicating a tenfold increase in the concentration of the city’s primary pollutant shortly after residents started bursting firecrackers.

Between 9pm and 10pm, which falls within the stipulated window provided by the Supreme Court for bursting firecrackers, the level of atmospheric PM2.5 almost doubled, according to the Central Pollution Control Board (CPCB) data. In 2017, Gurugram recorded an AQI of 398 the day after Diwali. “This year’s post-Diwali AQI might seem like a marginal improvement, but there was no ban on firecrackers last year and implementation of the Graded Response Action Plan was also not so stringent,” said Kuldip Singh, regional officer (Gurugram), Haryana State Pollution Control Board.

Singh also said that Gurugram recorded slower winds and lower temperatures around Diwali this year. “Last year, the festival was celebrated October 19, when meteorological conditions are more favourable, in terms of reducing pollution. So while there has certainly been a spike in pollution today (Thursday), compared to last year, the situation is not so bad,” he added. Experts agree, but also said that there was no doubt that Thursday’s pollution spike caused due to the widespread use of firecrackers. “Since yesterday was Diwali, emissions from industrial and vehicular sources would have been minimal, leaving only firecrackers as the most reasonable explanation for the increase in airborne pollutants,” said Dipankar Saha, a former member of the Central Pollution Control Board’s air quality lab.

Various private air quality monitors across the city also recorded a hazardous level of pollutants on Thursday. According to a private monitor in DLF-5, the AQI in the region was 377 at 6pm on Thursday, with PM2.5 at 327μg/m3. Another monitor in Sector 4 recorded an AQI of 421 (‘severe) at the same time, with PM2.5 at 520μg/m3. Data gathered by a group of independent researchers on Wednesday night reflected PM2.5 levels as high as 1,200μg/m3 in Sector 81.

The average 24-hour measure of PM2.5 stood at 407ug/m3 on Thursday at 7pm, up from 212ug/m3 on Wednesday. The safe limit, according to the National Ambient Air Quality Standards, is 60ug/m3.

Moreover, these pollutants were unable to disperse due to extremely slow wind speeds, which in Gurugram were as low as 1.7kmph in certain areas on Thursday. Wind speeds of at least 10kmph are required for effective dispersal of PM2.5 and PM10.

“However, winds are expected to pick up speed to around 4kmph tomorrow (Friday), which might improve the situation,” said Sachin Panwar, a city-based air quality scientist. According to the ministry of environment, forest and climate change’s system of air quality and weather forecasting and research (SAFAR), the forecasted AQI for Gurugram on Friday is 162 (‘moderate’).

“Wind patterns this season have been erratic and unlike what we’ve seen in the past years,” said Laveesh Bhandari, a Delhi-based environmental policy expert, referring to the phenomenon of western disturbances, which have been causing strong winds to blow over India’s north-west, which in turn help translocate airborne pollutants.

“While the exact behaviour of these winds has not been modelled, they have, on the whole, helped keep pollution levels from becoming much worse than last year in the Delhi-NCR,” he added, saying that a spike in PM2.5 from 60ug/m3 to 622ug/m3 in Gurugram was alarming, but not significant when compared with other cities in the NCR, which were worse off on Thursday.